Seminar report On “REHABILATATION OF SEISMICALLY DAMAGED STRUCTURES
” SUBMITTED TO VIVESWARAIAH TECHNOLOGICAL UNIVERSITY BELGAUM FOR THE PARTIAL FULFILLMENT OF M-TECH (STRUCTURAL ENGINEERING)
BY TARA SEN Reg. No: 1st Semester M-Tech Structures Under The Guidance of: H.N JAGANATHA REDDY Asst. Professor Department of Civil Engineering
BANGALORE INSTITUTE OF TECHNOLOGY (Affiliated To Visveswaraiah Technological University) Bangalore-560004
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BANGALORE INSTITUTE OF TECHNOLOGY BANGALORE -560004
CERTIFICATE This is to certify that MISS TARA SEN bearing university USN has submitted the seminar report on “REHABILATATION OF SEISMICALLY DAMAGED STRUCTURES ” in partial fulfillment of the 1st semester M-Tech course in structural engineering as prescribed by the Visveswaraiah Technological University during the academic year 2006-2007, under the guidance of H.N JAGANATHA REDDY
Prof. K.JAYRAM H.O.D Dept. of Civil Engg.
H.N JAGANATHA REDDY Asst Professor Dept. of Civil Engg.
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ACKNOWLEDGEMENT
I express my deep sense of gratitude to H.N JAGANATHA REDDY asst professor Department of Civil Engineering, BIT, for his guidance and help through out this seminar work. Without his support this seminar would have not been possible.
I will remain thankful to the head of department, PROF. K.JAYRAM and all the faculty members of Department of Civil Engineering, BIT for their support during the course of this work. Finally I express gratitude to my parents, fellow students and friends.
TARA SEN M.TECH STRUCTURES BANGALORE INSTITUTE OF TECHNOLOGY
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SYNOPSIS
Even though, the Code of Practice on Earthquake Resistant Design of Buildings and Structures is in existence since 1962, it is being followed only by few government organizations, as a result non compliant buildings are being constructed in the country specially in private sector. Only recently, the codal provisions on Earthquake Resistant Design are made mandatory in few States and its implementation is yet to take full momentum. As a result, existing earthquake unsafe buildings are still growing to an alarming proportion. Like other earthquakes in the past, the recent earthquakes of Killari 1993, Bhuj 2001, and Kashmir 2005 have exposed the seismic vulnerability of construction practices being followed in the country. It has clearly demonstrated that not only non-engineered rural houses are vulnerable to earthquakes; the so-called engineered multistoreyed buildings in big cities are also mostly vulnerable due to faulty design and construction. Considering the large number of people, high fatality in RC buildings and volume of economic activities, the social risk involved in cities is also very high; the seismic retrofitting of the existing buildings has to be undertaken to make these unsafe houses safe to resist future earthquakes, thereby reducing the number of casualties significantly. The problem of seismic retrofitting of large stock of unsafe buildings is so big that any government action is just not feasible and therefore individual house owner/builder has to undertake the retrofitting measures. However, government can take up retrofitting of its own buildings and some public utility buildings which are of post earthquake importance. Evaluation and rehabilitation of damaged structures is an urgent task after an earthquake, as safe shelter is under pressing demand after a damaging earthquake. This requires some quick evaluation and retrofitting techniques. The techniques for quick evaluation of need and viability of retrofitting, temporary emergency support of the damaged structures, and repair and retrofitting of structures are also covered.
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A lot of development has taken place in the area of evaluation and retrofitting of existing structures and it is still developing. These Guidelines cover the techniques suitable for the type of construction prevailing in India and which have been widely used and accepted to be safe. However, there are some more advanced methods of evaluation and retrofitting techniques, which are still at development stage. References have been provided for further study on these techniques. It is hoped that this will provide adequate information required for effective implementation of seismic retrofitting.
Structural Failure due to an earthquake
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WHY IS REHABILITATION DONE. The deficiencies in buildings and structures against earthquake may arise at (i) planning stage with faulty configuration and irregularities, (ii) design stage due to inadequate strength and ductility, and (iii) construction stage due to faulty construction practices. Revision of design codes is a continuing process world over and usually results in upgradation of seismic hazard and increase in design forces. In India also several regions have been upgraded in terms of seismic zones thereby rendering buildings unsafe according to new code. All these factors make the retrofitting of existing structures necessary. The retrofitting may also be required if change in usage of a building takes place or there is a major alteration/extension of building. LEVEL OF REHABILITATION The level of retrofitting of a building depends on the seismic zone in which building is situated and the level of performance desired from the building. Important buildings are desired to have a higher performance level during future earthquakes. The seismic zone governs the design earthquake forces and the performance level governs the permissible damage or the permissible values of member actions due to earthquake forces. Not only member forces and strength are important, the nonlinear deformations and ductile capacity of members are also important for seismic safety of building and need to be evaluated and examined.
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Much literature on retrofitting of building is already available including the Bureau of Indian Standards (BIS). The aim of present Guidelines is to provide an overview of the available techniques for seismic evaluation and retrofitting of existing buildings. The techniques have been presented for the type of construction prevailing in India. Emphasis has been on detailing the techniques with illustrations, so that these may be easily understood and applied by common engineers, architects and builders. A need has been felt to provide adequate information about seismic retrofit design of masonry and RC buildings which can be easily understood and implemented. The Guidelines deal with important aspects of seismic hazard estimation, systematic inspection of existing buildings, tests for estimation of in-situ strength and extent of damage and deterioration in masonry and RC buildings, mathematical modeling of frames, frame-tubes, shear walls and frames with infills, and various methods of analysis for earthquake forces for seismic evaluation, seismic evaluation which requires knowledge of structural behaviour, materials of construction, principles of seismic intervention and behaviour of modified structure, and various retrofitting materials. This includes performance levels of various types of buildings. The definition of these performance levels has been taken from FEMA and ATC. These Guidelines cover retrofitting of non-engineered, engineered and earthquake damaged buildings. These also cover non-engineered rural and semi-urban houses. These buildings are constructed in mud, stone or brick masonry, without any consideration to strength and ductility of the structure. The retrofitting techniques for such buildings are based on failure mode identification and behaviour of such buildings in past earthquakes. The techniques have been tested in laboratories and field, and known to provide adequate safety intended for such buildings.
REHABILITATION OBJECTIVE
Li fe S af 8 et Deflection y
Collapse Ductily
stifness
S er vi c e a
strength
Inelastic Range
Elastic Range
Coll aps e Pre ven tioN
L at er al L o a d
lit y
It is very evident from the graph that if the deflection vs the lateral load curve is with in the elastic range then only the stiffness of the structure should be increased by rehabilitation so as to enhance its serviceability. Now if the deflection vs the lateral load curve goes beyond the elastic range but is with in the inelastic range then both the stiffness and the strength of the structure has to be enhanced by rehabilitation so as to enhance both its serviceability as well as its life. Now if the deflection vs the lateral load curve goes beyond the inelastic range and reaches the collapse range then the stiffness, the strength and ductility should be increased by rehabilitation so as to enhance both its serviceability as well as its strength and to prevent it from collapsing.
GOALS OF REHABILITATION :
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Seismic retrofit is primarily applied to achieve public safety, with various levels of structure and material survivability determined by economic considerations: Public safety only. The goal is to protect human life, ensuring that the structure will not collapse upon its occupants or passers by, and that the structure can be safely exited. Under severe seismic conditions the structure may be a total economic write-off, requiring tear-down and replacement. Structure survivability. The goal is that the structure, while remaining safe for exit, may require extensive repair (but not replacement) before it is generally useful or considered safe for occupation. This is typically the lowest level of retrofit applied to bridges. Structure usability. The structure is to be undiminished in its utility, although it may be necessary to perform extensive repair or replacement of components in preparation for the next major seismic event. This is typically the lowest level of retrofit applied to fire fighting stations, public safety (police) command centers, and the like and is often the most economical level of retrofit and design for transportation infrastructure such as rail and highway roadways, bridges, and tunnels. This level of retrofit is required for water supplies used for fire fighting reservoirs, water lines, and hydrants, and is also needed for a few hours after a seismic event for household water supplies, which may be used for emergency fire fighting.
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Primary structure undamaged and the structure is undiminished in utility for its primary application. A high level of retrofit, this ensures that any required repairs are only "cosmetic" - for example, minor cracks in plaster, drywall and stucco. This is the minimum acceptable level of retrofit for hospitals. Structure unaffected. This level of retrofit is preferred for historic structures of high cultural significance. usually incorporate ductile steel frames and by their height will have lower natural frequencies.
REHABILITATION STRATEGY DESIGN PARAMETERS : Strength Stiffness Ductility
Base shear Base shear
Ductility enhancement rehabilitated Stiffness & Strength enhancement structure
rehabilitated structure existing structure ÄV Äu displacement existing structure
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Base shear
Stiffness , Strength and Ductility enhancement
rehabilitated structure
Stiffness, Strength & Ductility enhancement ÄV
Base shear
existing rehabilitated structure structure Äu displacement
existing structure
REHABILITATION INTERVENTION TECHNIQUES
Äu
11displacement
ÄV
They are of three different types: Local Intervention :
Such as injection into the cracks Gunite concreting Steel plate adhesion Steel jacketing FRP jacketing
Global Intervention : RC Jacketing Addition of RC walls Steel buttresses Steel Bracing Base Isolation Selective Intervention: Stiffness Increment Stiffness and Strength Increment Stiffness, Strength and Ductility Increment
DIFFERENT PROCESSES OF REHABILITATION FOR VARIOUS CASES OF STRUCTURE FAILURES DUE TO EARTHQUAKES 1. Reinforcement The most common form of seismic retrofit to lower buildings is adding strength to the existing structure to resist seismic forces. The strengthening may be limited to connections between existing building elements or it may involve adding primary resisting elements such as walls or frames, particularly in the lower stories. Connections between buildings and their expansion additions Frequently, building additions will not be strongly connected to the existing structure, but simply placed adjacent to it, with only minor continuity in flooring, siding, and roofing. As a result, the addition may have a different resonant period than the original structure, and they may easily detach from one another. The relative motion will then cause the two parts to collide, causing severe structural damage. Proper construction will tie the two building components rigidly together so that they behave as a single mass. Failure in lowest storey
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In many buildings the ground level is designed for different uses than the upper levels. Low rise residential structures may be built over a parking garage which have large doors on one side. Hotels may have a tall ground floors to allow for a grand entrance or ballrooms. Office buildings may have stores in the ground floor which desire continuous windows for display. Traditional seismic design assumes that the lower stories of a building are stronger than the upper stories and where this is not the case the structure will not respond to earthquakes in the expected fashion. Using modern design methods, it is possible to take a weak story into account. Typically, where this type of problem is found, the weak story is reinforced to make it stronger than the floors above by adding shear walls or moment frames. Moment frames consisting of inverted U bents are useful in preserving lower story garage access, while a lower cost solution may be to use shear walls or trusses in several locations, which partially reduce the usefulness for automobile parking but still allow the space to be used for other storage. Wooden floor failure
Floors in wooden buildings are usually constructed upon relatively deep spans of wood, called joists, covered with a diagonal wood planking or plywood to form a subfloor upon which the finish floor surface is laid. In many structures these are all aligned in the same direction. To prevent the beams from tipping over onto their side, blocking is used at each end, and for additional stiffness, blocking or diagonal wood or metal bracing may be placed between beams at one or more points in their spans. At the outer edge it is typical to use a single depth of blocking and a perimeter beam overall. If the blocking or nailing is inadequate, each beam can be laid flat by the shear forces applied to the building. In this position they lack most of their original strength and the 13
structure may further collapse. As part of a retrofit the blocking may be doubled, especially at the outer edges of the building. It may be appropriate to add additional nails between the sill plate of the perimeter wall erected upon the floor diaphragm, although this will require exposing the sill plate by removing interior plaster or exterior siding. As the sill plate may be quite old and dry and substantial nails must be used, it may be necessary to pre-drill a hole for the nail in the old wood to avoid splitting. When the wall is opened for this purpose it may also be appropriate to tie vertical wall elements into the foundation using specialty connectors and bolts glued with epoxy cement into holes drilled in the foundation. Sliding off foundation and cripple wall failure
House slid off of foundation
Low cripple wall collapse and detachment of structure from concrete stairway Single or two story wood-frame domestic structures built on a perimeter or slab foundation are relatively safe in an earthquake, but in many structures built before 1950 the sill plate that sits between the concrete foundation and the floor diaphragm (perimeter foundation) or studwall (slab foundation) may not be sufficiently bolted in. Additionally, older attachments may have corroded to a point of weakness. A sideways shock can also slide the building entirely off of the foundations or slab. Often such buildings, especially if constructed on a moderate slope, are erected on a platform connected to a perimeter foundation through low stud-walls called "cripple wall" or pin-up. This low wall structure itself may fail in shear or in its connections to 14
itself at the corners, leading to the building moving diagonally and collapsing the low walls. The likelihood of failure of the pin-up can be reduced by ensuring that the corners are well reinforced in shear and that the shear panels are well connected to each other through the corner posts. This requires structural grade sheet plywood, often treated for rot resistance. This grade of plywood is made without interior unfilled knots and with more, thinner layers than common plywood. New buildings designed to resist earthquakes will typically use OSB (oriented strand board), sometimes with metal joins between panels, and with well attached stucco covering to enhance its performance. In many modern tract homes, especially those build upon expansive (clay) soil the building is constructed upon a single and relatively thick monolithic slab, kept in one piece by high tensile rods that are stressed after the slab has set. This poststressing places the concrete under compression - a condition under which it is extremely strong in bending and so will not crack under adverse soil conditions. Multiple piers in shallow pits Some older low-cost structures are elevated on tapered concrete pylons set into shallow pits, a method frequently used to attach outdoor decks to existing buildings. This is seen in conditions of damp soil, especially in tropical conditions, as it leaves a dry ventilated space under the house, and in far northern conditions of permafrost (frozen mud) as it keeps the building's warmth from destabilizing the ground beneath. During an earthquake, the pylons may tip, spilling the building to the ground. This can be overcome by using deep-bored holes to contain cast-in-place reinforced pylons, which are then secured to the floor panel at the corners of the building. Another technique is to add sufficient diagonal bracing or sections of concrete shear wall between pylons. Reinforced concrete column burst
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Jacketed and grouted column on left, unmodified on right Reinforced concrete columns typically contain large diameter vertical rebar arranged in a ring, surrounded by lighter-gauge hoops of rebar. Upon analysis of failures due to earthquakes, it has been realized that the weakness was not in the vertical bars, but rather in inadequate strength and quantity of hoops. Once the integrity of the hoops are breached, the vertical rebar can flex outward, stressing the central column of concrete. The concrete then simply crumbles into small pieces, now unconstrained by the surrounding rebar. In new construction a greater amount of hoop-like structures are used. One simple retrofit is to surround the column with a jacket of steel plates formed and welded into a single cylinder. The space between the jacket and the column is then filled with concrete, a process called grouting. Where soil or structure conditions require such additional modification, additional pilings may be driven near the column base and concrete pads linking the pilings to the pylon are fabricated at or below ground level. In the example shown not all columns needed to be modified to gain sufficient seismic resistance for the conditions expected.
Brick wall resin and glass fiber reinforcement
Brick building structures have been reinforced with coatings of glass fiber and appropriate resin (epoxy or polyester). In lower floors these may be applied over entire exposed surfaces, while in upper floors this may be confined to narrow areas around window and door openings. This application provides tensile strength that stiffens the wall against bending away from the side with the application. The efficient protection of an entire building requires extensive analysis and engineering to determine the appropriate locations to be treated.
Reinforced concrete post to beam connections
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EMBED Word.Picture.8 Corner joint steel reinforcement and high tensile strength rods with grouted anti-burst jacket below Examination of failed structures often reveals failure at the corners, where vertical posts join horizontal beams. These corners can be reinforced with external steel plates, which must be secured by through bolts and which may also offer an anchor point for strong rods, as shown in the image at left. The horizontal rods pass across the beam to a similar structure on the opposite side, while the vertical rods are anchored after passing through a grouted anti-burst jacket.
Original construction with well connected double posts and saddle beam Another method is to simply add a great amount of small attachment points, as in the wall reinforcement method described above, with additional rebar and concrete. In one retrofit every corner joint has been surrounded by a block-like jacket. These blocks serve to transfer bending forces to new added jackets on the vertical and horizontal elements. The goal is to achieve the type of strength afforded by the new construction shown at right (this is not a retrofit). 2. Use of Ferrocement
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Use of Ferrocement In India, ferrocement is used often because the constructions made from it are better resistant against earthquakes. Advantages The advantages of a well built ferrocement construction are the low weight, maintenance costs and long lifetime in comparison with steel constructions. However, meticulous building precision is considered crucial here. Especially with respect to the cement composition and the way in which it is applied in and on the framework. When a ferrocement sheet is mechanically overloaded, it will tend to fold instead of crack or rupture. The wire framework will hold the pieces together, which in some applications (boat hull, ceiling, roof) is an advantage.
3. Isolation
Generally required for large masonry buildings, excavations are made around the foundations of the building and the building (in piecemeal fashion) is separated from the foundations. Steel or reinforced concrete beams replace the connections to the foundations, while under these, layered rubber and metal isolating pads replace the material removed, these in turn are attached below to new or existing foundations. These allow the ground to move while the building, restrained by its inertial mass, remains relatively static. The pads absorb energy, transforming the relative motion between the ground and the structure into heat. While the pads tend to transmit some of the ground motion to the building they also keep the building positioned properly over the foundation. Careful attention to detail is required where the building interfaces with the ground, especially at entrances, stairways and ramps, to ensure sufficient free motion
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without damage to access means from compression or dismantling or falling from extension. 4. Dampers
Dampers absorb the energy of motion and convert it to heat, thus "damping" resonant effects in structures that are rigidly attached to the ground. In these cases, the threat of damage does not come from the initial shock itself, but rather from the periodic resonant motion of the structure that repeated ground motion induces. 5.Slosh tanks
A large tank of water may be placed on an upper floor. During a seismic event, the water in this tank will slosh back and forth, but is directed by baffles - partitions that prevent the tank itself becoming resonant; through its mass the water may change or counter the resonant period of the building. Additional kinetic energy can be converted to heat by the baffles and is dissipated through the water - any temperature rise will be insignificant. 6.Shock absorbers
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Shock absorbers, similar to those used in automotive suspensions, may be used to connect portions of a structure that are free to move relative to each other and that may collide during an earthquake. Where a rigid connection could break or impose excessive strain on the buildings, and a loose connection could be dismantled, the shock absorbers allow the relative motion to be restrained by transferring and dissipating energy. This can be especially effective if the two structures have differing fundamental frequencies of resonance, as each structure may then assist in inhibiting the motion of the other. 7.Tuned mass dampers
Tuned mass dampers employ movable weights with dampers. These are typically employed to reduce wind sway in very tall, light buildings. Similar designs may be employed to impart earthquake resistance in eight to ten story buildings that are prone to destructive earthquake induced resonances. 8.Active damping with fallback
20 Landmark Tower. Shinjuku Park
Very tall buildings ("skyscrapers"), when built using modern lightweight materials, might sway uncomfortably (but not dangerously) in certain wind conditions. A solution to this problem is to include at some upper story a large mass, constrained, but free to move within a limited range, and moving on some sort of bearing system such as an air cushion or hydraulic film. Hydraulic pistons, powered by electric pumps and accumulators, are actively driven to counter the wind forces and natural resonances. These may also, if properly designed, be effective in controlling excessive motion - with or without applied power - in an earthquake. In general, though, modern steel frame high rise buildings are not as subject to dangerous motion as are medium rise (eight to ten story) buildings, as the resonant period of a tall and massive building is longer than the approximately one second shocks applied by an earthquake. 9.Rehabilitation of natural gas lines : Natural gas and propane supply pipes to structures often prove especially dangerous during and after earthquakes. Should a building move from its foundation or fall due to cripple wall collapse, the ductile iron pipes transporting the gas within the structure may be broken, typically at the location of threaded joints. The gas may then still be provided to the pressure regulator from higher pressure lines and so continue to flow in substantial quantities; it may then be ignited by a nearby source such as a lit pilot light or arcing electrical connection. There are two primary methods of automatically restraining the flow of gas after an earthquake, installed on the low pressure side of the regulator, and usually downstream of the gas meter. • A caged metal ball may be arranged at the edge of an orifice. Upon seismic shock, the ball will roll into the orifice, sealing it to prevent gas flow. The ball may later be reset by the use of an external magnet. This device will respond only to ground motion.
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A flow-sensitive device may be used to close a valve if the flow of gas exceeds a set threshold (very much like an electrical circuit breaker). This device will operate independently of seismic motion, but will not respond to minor leaks which may be caused by an earthquake.
It appears that the most secure configuration would be to use one of each of these devices in series.
DIFFERENT TECHNIQUES OF REHABILITATION
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RC Jacketing
Local intervention
Side Jacketing
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Using Fiber-reinforced concrete Fiber-reinforcement is mainly used in shotcrete, but can also be used in normal concrete. Fiber-reinforced normal concrete are mostly used for on-ground floors and pavements, but can be considered for a wide range of construction parts (beams, pilars, foundations etc) either alone or with hand-tied rebars. Concrete reinforced with fibers (which are usually steel or "plastic" fibers) is less expensive than hand-tied rebar, while still increasing the tensile strength many times. Shape, dimension and length of fiber is important. A thin and short fiber, for example short hair-shaped glass fiber, will only be effective the first hours after pouring the concrete (reduces cracking while the concrete is stiffening) but will not increase the concrete tensile strength. A normal size fibre for European shotcrete (1 mm diameter, 45 mm length—steel or "plastic") will increase the concrete tensile strength. Steel is the strongest commonly-available fiber, and come in different lengths (30 to 80 mm in Europe) and shapes (end-hooks). Steel fibres can only be used on surfaces that can tolerate or avoid corrosion and rust stains. In some cases, a steel-fiber surface is faced with other materials. Glass fiber is inexpensive and corrosion-proof, but not as ductile as steel. Recently, spun basalt fiber, long available in Eastern Europe, has become available in the U.S. and Western Europe. Basalt fibre is stronger and less expensive than glass, but historically, has not resisted the alkaline environment of portland cement well enough to be used as direct reinforcement. New materials use plastic binders to isolate the basalt fiber from the cement. The premium fibers are graphite reinforced plastic fibers, which are nearly as strong as steel, lighter-weight and corrosion-proof. Some experimeters have had promising early results with carbon nanotubes, but the material is still far too expensive for any building
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FRP jacketing Columns
Beams
Shear Strengthening
Column Wraping device
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Large capacity isolation bearings and dampers
Wall Failures
Using of steel bolts for retrofitting of walls
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Using steel braces
STRUCTURES TO BE REHABILITATED : Seismic rehabilitation techniques will vary with the nature of the structure, soil conditions, local topography, and distance from various faults. A nearby minor fault, capable of generating only a small earthquake, may be more dangerous to a structure than a distant major fault. In some cases, structures have been built spanning faults, and an appropriate retrofit may be to attempt to keep the portions together or to remove or make a spanning portion flexible. 1.Bridges Bridges have several failure modes.They are the following : Expansion joints Many short bridge spans are statically anchored at one end and attached to joints at the other. This joints gives vertical and transverse support while allowing the bridge span to expand and contract with temperature changes. The change in the length of the span is accommodated over a gap in the roadway by comb-like expansion joints. During severe ground motion the rockers may jump from their tracks or be moved beyond their design limits, causing the bridge to unship from its resting point and then either become misaligned or fall completely. Motion can be constrained by adding ductile or high-strength steel restraints that are friction-clamped to beams and designed to slide under stress while limiting the motion relative to the anchorage.
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Lattice beams Obsolete riveted lattice beams shown in the figure. Lattice beams consist of two "I"-beams connected with a criss-cross lattice of flat strap or angle stock. These can be greatly strengthened by replacing the open lattice with plate members. This is usually done in concert with the replacement of hot rivets with bolts.
Bolted plate lattice replacement, forming box beams Hot rivets Many older structures were fabricated by inserting red hot rivets into pre-drilled holes; the rivets are then peened using an air hammer on one side and a bucking bar (an inertial mass) on the head end. As these cool slowly, they are left in an annealed (soft) condition,
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while the plate, having been hot rolled and quenched during manufacture, remains relatively hard. Under extreme stress the hard plates can shear the soft rivets, resulting in failure of the joint. The solution is to burn out each rivet with an oxygen torch. The hole is then prepared to a precise diameter with a reamer. A special bolt, consisting of a head, a shaft matching the reamed hole, and a threaded end is inserted and retained with a nut, then tightened with a wrench. As the bolt has been formed from an appropriate high-strength alloy and has also been heat-treated, it is not subject to either the plastic shear failure typical of hot rivets nor the brittle fracture of ordinary bolts. Any partial failure will be in the plastic flow of the metal secured by the bolt; with proper engineering any such failure should be noncatastrophic. 2. Tunnels
Unless the tunnel penetrates a fault likely to slip, the greatest danger to tunnels is a landslide blocking an entrance. Additional protection around the entrance may be applied to divert any falling material (similar as is done to divert snow avalanches) or the slope above the tunnel may be stabilized in some way. Where only small- to medium-sized rocks and boulders are expected to fall, the entire slope may be covered with wire mesh, pinned down to the slope with metal rods. This is also a common modification to highway cuts where appropriate conditions exist. 3. Underwater tubes The safety of underwater tubes is highly dependent upon the soil conditions through which the tunnel was constructed, the materials and reinforcements used, and the maximum predicted earthquake expected, and other factors, some of which may remain unknown under current knowledge. A tube of particular structural, seismic, economic, and political interest is the BART (Bay Area Rapid Transit) trans-bay tube. This tube was constructed at the bottom of San Francisco Bay through an innovative process. Rather than pushing a shield through the soft bay mud, the tube was constructed on land in sections. Each section consisted of two inner tubular tunnels, a central access tunnel of rectangular cross section, and an outer oval shell encompassing the three inner tubes. The intervening space was filled with concrete. At the bottom of the bay a trench was excavated and a flat bed of crushed stone prepared to receive the tube sections. The sections were then floated into place and sunk, then joined with bolted connections to previously-placed sections. An overfill was then placed atop the tube to hold it down. Once completed from San Francisco to Oakland, the 29
tracks and electrical components were installed. The predicted response of the tube during a major earthquake was likened to be as that of a string of (cooked) spaghetti in a bowl of gelatin dessert). To avoid overstressing the tube due to differential movements at each end, a sliding slip joint was included at the San Francisco terminus under the landmark Ferry Building. The engineers of the construction consortium PBTB (ParsonsBrinkerhoff-Tudor-Bechtel) used the best estimates of ground motion available at the time, now known to be insufficient given modern computational analysis methods and geotechnical knowledge. Unexpected settlement of the tube has reduced the amount of slip that can be accommodated without failure. These factors have resulted in the slip joint being designed too short to ensure survival of the tube under possible (perhaps even likely) large earthquakes in the region. To correct this deficiency the slip joint must be extended to allow for additional movement, a modification expected to be both expensive and technically and logistically difficult. Other retrofits to the BART tube include vibratory consolidation of the tube's overfill to avoid potential liquefying of the overfill, which has now been completed. (Should the overfill fail there is a danger portions of the tube rising from the bottom, an event which could potentially cause failure of the section connections.) 4.Fill and overpass
Elevated roadways are typically built on sections of elevated earth fill connected with bridge-like segments, often supported with vertical columns. If the soil fails where a bridge terminates, the bridge may become disconnected from the rest of the roadway and break away. The retrofit for this is to add additional reinforcement to any supporting wall, or to add deep caissons adjacent to the edge at each end and connect them with a supporting beam under the bridge. Another failure occurs when the fill at each end moves (through resonant effects) in bulk, in opposite directions. If there is an insufficient founding shelf for the overpass it may then fall. Addtional shelf and ductile stays may be added to attach the overpass to the footings at one or both ends. The stays, rather than being fixed to the beams may instead be clamped to them. Under moderate loading these keep the overpass centered in the gap so that it is less likely to slide off its founding shelf at one end. The ability for the fixed ends to slide, rather than break, will prevent the complete drop of the structure If it should fail to remain on the footings. 30
5.Viaducts Large sections of roadway may consist entirely of viaduct, sections with no connection to the earth other than through vertical columns. When concrete columns are used, the detailing is critical. Typical failure may be in the toppling of a row of columns due either to soil connection failure or to insufficient cylindrical wrapping with rebar. Both failures were seen in the 1995 Great Hanshin earthquake in Kobe, Japan, where an entire viaduct, centrally supported by a single row of large columns, was laid down to one side. Such columns are reinforced by excavating to the foundation pad, driving additional pilings, and adding a new, larger pad, well connected with rebar along side of or into the column. A column with insufficient wrapping bar, which is prone to burst and then hinge at the bursting point, may be completely encased in a circular or elliptical jacket of welded steel sheet and grouted as described above.
Cypress Freeway viaduct collapse. Note lack of anti-burst wrapping and lack of connection between upper and lower vertical elements. Sometimes viaducts may fail in the connections between components. This was seen in the failure of the Cypress Freeway in Oakland, California during the Loma Prieta earthquake. This viaduct was a two-level structure, and the upper portions of the columns were not well connected to the lower portions that supported the lower level; this caused the upper deck to collapse upon the lower deck. Weak connections such as these require additional external jacketing - either through external steel components or by a complete jacket of reinforced concrete, often using stub connections that are glued (using epoxy adhesive) into numerous drilled holes. These stubs are then connected to additional wrappings, external forms (which may be temporary or permanent) are erected, and additional concrete is poured into the space. Large connected structures similar to the Cyprus Viaduct must also be properly analyzed in their entirety using dynamic computer simulations. 6.Retaining Walls
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Concrete walls are often used at the transition between elevated road fill and overpass structures. The wall is used both to retain the soil and so enable the use of a shorter span and also to transfer the weight of the span directly downward to footings in undisturbed soil. If these walls are inadequate they may crumble under the stress of an earthquake's induced ground motion. One form of retrofit is to drill numerous holes into the surface of the wall, and secure short L-shaped sections of rebar to the surface of each hole with epoxy adhesive. Additional vertical and horizontal rebar is then secured to the new elements, a form is erected, and an additional layer of concrete is poured. This modification may be combined with additional footings in excavated trenches and additional support ledgers and tie-backs to retain the span on the bounding walls. 32
6.Wood frame
Wood is one of the best materials for anti-seismic construction since it is of low mass and is relatively less brittle than masonry. It is easy to work with and very cheap compared to other modern material as steel and reinforced concrete. This is only resistant if the structure is properly connected to its foundation and has adequate shear resistance, in modern construction obtained by well connected surfacing of panels with plywood or oriented strand board in combination with exterior stucco. Steel strapping and sheet forms are also used to connect elements securely. Retrofit methods in older frame structures may consist of the following, and other methods not described here. • The lowest plate rails of walls are bolted to a continuous foundation, or held down with rigid metal clips bolted to the foundation. • Selected vertical elements, especially at wall junctures and window and door openings are attached securely to the sill plate. • In two story buildings using "western" style construction (walls are progressively erected upon the lower story's upper diaphragm, unlike "eastern" balloon framing), the upper walls are connected to the lower walls with tension elements. In some case connections may be extended vertically to include retention of certain roof elements. • Low cripple walls are made shear resistant by adding plywood at the corners and by securing corners from opening with metal strapping or fixtures. • Vertical posts may be restrained from jumping off of their footings. 7.Masonry In the western United States, much of what is seen as masonry is actually brick or stone veneer. Current construction rules dictate the amount of tie–back required, which consist of metal straps secured to vertical structural elements. These straps extend into mortar courses, securing the veneer to the primary structure. Older structures may not secure this sufficiently for seismic safety. A weakly secured veneer in the interior house (sometimes used to face a fireplace from floor to ceiling) can be especially dangerous to occupants.
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Older masonry chimneys are also dangerous if they have substantial vertical extension above the roof. These are prone to breakage at the roofline and may fall into the house in a single large piece. For retrofit, additional supports may be added or it may be better to simply remove the extension and replace it with lighter materials, with special piping replacing the flue tile and a wood structure replacing the masonry. This may be matched against existing brickwork by using very thin veneer (similar to a tile, but with the appearance of a brick). Masonry walls that are not reinforced are especially hazardous. Such structures may be more appropriate for replacement than retrofit, but if the walls are the principal load bearing elements in structures of modest size they may be appropriately reinforced. It is especially important that floor and ceiling beams be securely attached to the walls. Additional vertical supports in the form of steel or reinforced concrete may be added. 8.RC BUILDING Retrofitting of RC buildings is much more systematic and rational process than that of non-engineered load bearing wall buildings. The different techniques available for retrofitting of RC buildings have been described. The principles of retrofitting of RC buildings are: (i) removal of irregularities and asymmetry, (ii) Increasing the strength and stiffness of structure, (iii) Enhancement of deformation capacity (or ductility), and (iv) Earthquake demand reduction by Base-isolation or Supplemental Energy Dissipation. Different techniques based on these principles have been illustrated. The emphasis on reinforcement detailing, bond of old and new concrete, and anchorage of new reinforcement is highlighted. Outline and principle of advanced techniques (e.g. BaseIsolation and Supplemental Damping) has also been provided. However, a detailed description and mathematical formulation of these advanced techniques are beyond the scope of these Guidelines and references have been provided for further reference. Retrofitting and strengthening of existing structures require use of special materials. Bonding of old and new concrete and shrinkage are the main governing factors in selection of material. A description of materials available for this purpose, including a range from ordinary cement-sand grout, concrete to polymers and epoxy, use of Fibre Reinforced Polymers/Plastics (FRP) in strengthening and retrofitting has also been described with the points of caution. Specialized machinery and preparations required for use of different retrofitting materials are also outlined. ADVANTAGES OF RETROFFITING 1.Damage and collapse of bridges can be prevented with proper seismic design and detailing. 2.Understanding plastic concepts can produce more economical foundations.
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3. Utilizing the latest seismic guidelines produces safe and state-of-the-art designs. 4. Traffic congestion and interruption to commerce can be minimized if bridges remain in service following a seismic event. 5. Higher strength/weight ratio. 6. Higher oriented strength. 7. Better design flexibility. 8. Lower maintenance and long term durability. 9. Better dimensional stability. 10. Protection of various building components can be accomplished . 11. Will enable us in better protection of life and property. 12.Will help us to preserve various historic monuments for years to come. Release of hazardous materials with associated environmental impacts.
The End
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